Project Managing Your London Extension: What to Expect at Each Stage

The pre-start period — between signing the contract and the contractor arriving on site — is the most important preparation stage for the homeowner. Key items to confirm before the contractor mobilises on site: Planning permission confirmed: confirm in writing (email to the contractor and your architect) that the planning permission is in place and that any pre-commencement planning conditions have been discharged. Building Regulations approval: confirm either a Full Plans Approval (the preferred route — plans inspected and approved before work starts) or a Building Notice is in place. Party Wall Award: if the extension is adjacent to a party wall, confirm that the Party Wall Award has been served and agreed before excavation or structural works begin. Contract signed and deposit agreed: ensure the JCT contract (or agreed contract form) is signed by both parties and the deposit amount and payment stage structure are confirmed in writing before any payment is made. Contractor insurance confirmed: request copies of the contractor's public liability insurance certificate (minimum £2-5 million cover) and employer's liability insurance before mobilisation. Construction Method Statement: where required by a planning condition or by Thames Water (sewer build-over), confirm this is in place before groundworks start. Site access and neighbour communication: if the extension requires scaffolding over or adjacent to a neighbouring property, or if a crane or heavy plant is required, confirm the access arrangements and communicate with neighbours before works start. Party wall condition schedule: the appointed party wall surveyor should have prepared a Schedule of Condition of the neighbouring property before works begin — this is evidence of the existing condition against which any damage claim is assessed.

What happens: setting out the extension footprint; removal of existing paving, planting, or structures within the extension footprint; excavation for strip foundations, raft, or mini-piled foundations; installation of drainage diversions or new drainage; pouring the foundations and oversite slab. What the homeowner should check at this stage: Foundation inspection: Building Control will inspect the foundation excavation at formation level (before concrete is poured) — do not pour foundations until Building Control has visited and confirmed the excavation is acceptable. This is a mandatory hold point. Drainage locations: confirm the drainage layout before concrete is poured over drains — it is very difficult to correct drainage routing after the oversite slab is complete. DPC level: confirm the DPC (Damp Proof Course) is at the specified height above finished ground level (minimum 150mm above external ground level). This is a permanent decision — the DPC level affects how the extension sits relative to the existing house threshold and the external finished ground levels. Tree protection: where trees adjacent to the extension are covered by a Tree Preservation Order (TPO), ensure the contractor is working within the tree protection zone guidance — no storage of materials, no soil compaction, no chemical contamination within the tree root protection zone. Common mistakes at groundworks stage: pouring foundation concrete before Building Control inspection; incorrect drainage fall directions before concrete oversite covers the drains; DPC set too low, leading to damp at floor level.

What happens: building the masonry or timber frame structural walls; structural steel installation (RSJs/UC sections over new openings to the existing house, and for any spanning required within the extension); roof structure (timber pitched roof or flat roof build-up); roof covering (tiles, slates, lead flashings, or flat roof membrane). What the homeowner should check at this stage: Structural steel: Building Control will inspect structural steel beam positions and padstones before the beam is loaded (masonry built above it). Do not build above a new beam before Building Control inspection. Flashings at the existing wall junction: the junction between the new extension roof and the existing house wall is the most common source of leaks in London extensions. Confirm the flashing detail (lead soaker, lead cover flashing, or proprietary flashing) with the architect before the roofer installs the roof covering — get the detail right before the roof goes on. Wall tie positioning: where the new extension masonry is built off the existing party wall, wall ties connecting the new wall to the existing structure must be installed at the specified positions and centres. The structural engineer's specification for wall tie type and position must be followed. Roof insulation and VCL: the insulation specification and the position of the vapour control layer in a flat roof are permanently inaccessible once the roof build-up is complete. Confirm the roof build-up matches the specification before the roof covering is installed. Building Control stage inspection at roof level: the Building Control officer will inspect insulation, structure, and weatherproofing before the ceiling and any internal roof finish covers the structure.

What happens: first fix electrical wiring (circuits run in floors, walls, ceiling voids before plastering); first fix plumbing (soil pipes, waste pipes, underfloor heating pipework, boiler connections run in position before floor and wall finishes); heating pipework installation; any ventilation ductwork. What the homeowner should check at first fix M&E stage: Electrical circuit layout: confirm the position of sockets, switches, lighting points, and any specialist circuits (underfloor heating thermostat, extractor fan, EV charger, kitchen appliances) with your electrician before first fix is complete. It is far cheaper to re-route a first-fix cable before plastering than after. MVHR or mechanical ventilation ductwork: if mechanical ventilation with heat recovery (MVHR) is specified for the extension, the ductwork runs must be in position before ceiling and wall linings are installed — the MVHR is designed before first fix, not added at second fix. Soil and waste pipe falls: check the fall on all new waste pipes before they are concealed — waste pipes typically require a 1:40 to 1:80 fall to discharge correctly. Pipes with insufficient fall cause blockages. Underfloor heating manifold position: confirm the UFH manifold position and zone layout before the screed is poured — it is very difficult to alter the manifold position after the screed is complete. Building Control Part P: electrical first fix in a London extension is notifiable under Building Regulations Part P (Electrical Safety in Dwellings). The electrical contractor must be either a Part P registered electrician or the works must be separately inspected and certified by Building Control.

What happens: installation of insulated plasterboard (where wall insulation is integrated with the internal lining) or bonding plaster; float and set plaster finish; second fix electrics (fitting of sockets, switches, light fittings); second fix plumbing (sanitary ware, taps, radiators, kitchen appliances connected); flooring; decoration; kitchen and bathroom fit-out (if applicable); external works (paving, landscaping, drainage gully, steps). What the homeowner should check at second fix and completion: Snagging list: prepare a detailed snagging list at practical completion — systematically checking every room, surface, fixture, and fitting. Include photos of each defect. Common snagging items on London extensions: incomplete mastic sealing at junctions; incomplete decoration (missed areas around door frames, window reveals); windows or doors that do not open smoothly; damaged tiles; incomplete grouting; extractor fan not wired or ducted correctly; missing fixings or damaged skirting boards. Building Regulations completion certificate: do not accept practical completion until the Building Control completion certificate has been applied for (or the Building Control officer's final inspection has been carried out). The completion certificate is a legal document required for future sale, remortgage, or lease of the property. Test and commission all M&E: heating system commissioned by the plumber; electrical circuits signed off (EICR — Electrical Installation Condition Report) by the electrician; underfloor heating system balanced; extractor fans tested for correct airflow; MVHR system commissioned; hot water cylinder setpoint confirmed. Retention release: confirm the snagging list items have been remedied before releasing the retention — do not release retention before the snagging items are complete and the Building Control completion certificate is issued.